Protective layer for charged particle beam processing

ABSTRACT

A protective layer is applied to a work piece to protect the surface during charged particle beam processing by directing a fluid toward the surface. The surface is preferably not touched by the applicator. Ink jet print-type print heads are suitable applicators. Ink jet-type print heads allow a wide variety of fluids to be used to form the protective layer. Useful fluids that form protective layers include colloidal silica having small silver particles and hydrocarbon-based inks.

This application claims priority from U.S. Prov. App. No. 60/855,536,filed Oct. 31, 2006, which is hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the application of a protective layeronto a work piece surface to protect the surface duringcharged-particle-beam processing.

BACKGROUND OF THE INVENTION

Charged particles beams, such as ion beams and electron beams, are usedfor processing work pieces in nanotechnology because charged particlebeams can form very small spots. For example, focused ion beam systemsare able to image, mill, deposit, and analyze with sub-micron precision.Focused ion beam systems are commercially available, for example, fromFEI Company, Hillsboro, Oreg., the assignee of the present application.The ions can be used to sputter, that is, physically eject, materialfrom a work piece to produce features, such as trenches, in the workpiece. An ion beam can also be used to activate an etchant gas toenhance sputtering, or to decompose a precursor gas to deposit materialnear the beam impact point. An ion beam can also be used to form animage of the work piece, by collecting secondary particles ejected bythe impact of the ion beam. The number of secondary particles ejectedfrom each point on the surface is used to determine the brightness ofthe image at a corresponding point on the image. Focused ion beams areoften used in the semiconductor industry. In one application, forexample, a focused ion beams is used to cut a small trench into anintegrated circuit to expose a cross section of a vertical structure forobservation or measurement using an ion beam or an electron beam.

Electron beams can also be used to process a work piece. Electron beamprocessing is described, for example in U.S. Pat. No. 6,753,538 to Mucilet al. for “Electron Beam Processing.” Electron beams are more commonlyused for forming images in a process called electron microscopy.Electron microscopy provides significantly higher resolution and greaterdepth of focus than optical microscopy. In a scanning electronmicroscope (SEM), a primary electron beam is focused to a fine spot thatscans the surface to be observed. Secondary electrons are emitted fromthe surface as it is impacted by the primary beam. The secondaryelectrons are detected, and an image is formed, with the brightness ateach point of the image being determined by the number of secondaryelectrons detected when the beam impacts a corresponding point on thesurface.

In a transmission electron microscope (TEM), a broad electron beamimpacts the sample and electrons that are transmitted through the sampleare focused to form an image of the sample. The sample must besufficiently thin to allow many of the electrons in the primary beam totravel though the sample and exit on the opposite site. Samples aretypically thinned to a thickness of less than 100 nm. One method ofpreparing samples includes using a focused ion beam to cut a thin samplefrom a work piece, and then using the ion beam to thin the sample.

In a scanning transmission electron microscope (STEM), a primaryelectron beam is focused to a fine spot, and the spot is scanned acrossthe sample surface. Electrons that are transmitted through the workpiece are collected by an electron detector on the far side of thesample, and the intensity of each point on the image corresponds to thenumber of electrons collected as the primary beam impacts acorresponding point on the surface.

When a charged particle beam impacts a surface, there is the potentialfor damage or alteration of the surface. Focused ion beam systemstypically use gallium ions from liquid metal gallium ion source. Galliumions are relatively heavy, and a gallium ion accelerated through atypical 30,000 volts will inevitably alter the work piece surface.Plasma ion systems, such as the one described in WO20050081940 of Kelleret al. for a “Magnetically Enhanced, Inductively Coupled, Plasma Sourcefor a Focused Ion Beam System” can use lighter ions, which cause lessdamage, but the ions will still typically alter the work piece surface.Electrons, while much lighter than ions, can also alter a work surface.When a user desires to measure a work piece with an accuracy ofnanometers, changes in the work piece caused by the impact of chargedparticles can be significant, especially in softer materials, such asphotoresist and low and ultra-low-k dielectric materials, such aspolyphenylene materials.

To protect the work piece surface, it is common to apply a protectivelayer before charged-particle-beam processing. One method of applying aprotective layer is charged-particle-beam deposition, that is, using acharged-particle beam to provide energy to decompose a gas to deposit amaterial on the surface. The protective layer shields the area aroundthe cut and preserves the characteristics of the features that are to beimaged and measured. Commonly used deposition gasses include precursorcompounds that decompose to deposit tungsten, platinum, gold, andcarbon. For example, tungsten hexacarbonyl can be used to deposittungsten, methylcyclopentadienyl trimethyl platinum can be used todeposit platinum, and styrene can be used to deposit carbon. Precursorgases to deposit many different materials are known in the art. Thepreferred material to be deposited as a protective layer depends on theapplication, including the composition of the underlying target surface,and the interaction between the protective layer material and the targetsurface.

Although charged-particle-beam-assisted deposition can locally apply alayer at the precise location where the layer is needed, applying aprotective layer using charged particle beam deposition has severaldisadvantages. Charged-particle-beam-assisted deposition is relativelyslow and, in some processes, up to sixty percent of the total processingtime is consumed in deposition of the protective layer. When an ion beamis initially scanned onto the target surface to deposit material, thebeam sputters material away from the surface for an initial period oftime until a sufficient amount of deposition material accumulates toshield the surface from the ion beam. Even though that period of timemay be small, it can be large enough to allow a significant amount ofmaterial to be removed, which causes the accuracy of the cross-sectionalanalysis to be compromised.

Currently, technicians quantify the dimensional change that is caused bythe charged particle beam deposition of the protective layer, and thenapply a correction factor to subsequent measurements to obtain anestimate of the true dimension. Such estimates are not always accuratebecause of the variation in the alteration by the charged particle beam.When a user desires to use an ion beam to extract a sample viewing witha TEM, as described for example, in U.S. Pat. No. 5,270,552 to Ohnishi,et al. “Method for Separating Specimen and Method for Analyzing theSpecimen Separated by the Specimen Separating Method,” the usertypically scans the focused ion beam in an imaging mode to locate theregion of interest. The scanning causes damage to the surface. When theregion of interest is located and the beam begins to mill a trench,there is additional damage to the work piece because the edges of thebeam are not perfectly sharp. That is, the beam is typically Gaussianshaped, and the ions in the tail of the Gaussian distribution willdamage the work piece at the edge of the trench. Damage has been foundnot just on fragile materials, but also on relatively hard materials.

Electron and laser beams can be used to generate secondary electrons todecompose a precursor gas to deposit a protective layer, but those beamsmay also damage the underlying surface—especially when they are atsufficient energy and/or current density levels for achieving favorableprocessing time. It is normally not practical to use such beams becausedeposition will typically be too slow if the beams are “weak” enough notto harm the underlying surface. Physical vapor deposition (“PVD”)sputter methods could be used to deposit protective layers in someapplications, but they normally cannot be utilized for productioncontrol applications in wafer fabrication facilities because suchmethods cannot be used to locally apply a deposition layer onto atargeted part of the wafer surface. U.S. patent application Ser. No.11/706,053 of Schmidt et al. for “Sputtering Coating of Protective Layerfor Charged Particle Beam Processing” which is assigned to the assigneesof the present invention, describes a method of PVD that can provide alocalized layer. A charged-particle beam is used to sputter materialfrom a target onto the surface. The charged-particle beam is notdirected to the surface itself and damage is avoided. This method,however, is time consuming.

Another method of applying a protective coating is described is U.S.Pat. No. 6,926,935 to Arjavec et al. for “Proximity Deposition.” In thismethod, the charged particle beam is not directed at the area ofinterest, but to a region outside the area of interest. Secondaryelectrons decompose the precursor gas over the area of interest toprovide a protective layer. As the protective layer is being createdaround the edge of the region of interest, the charged particle beam canbe moved inward. This method is also time consuming.

Colloidal silver applied with a brush has long been used to produce aconductive protective layer in scanning electron microscopy. The silverparticles used are relatively large. Using a brush to apply the layercan damage the substrate and cannot provide a localized layer.

Another method of applying a protective coating is to use a felt tippen, such as a Sharpie brand pen from the Sanford division of Rubbermaidcorporation The ink from a Sharpie pen is suitable for use in a vacuumchamber, because it dries thoroughly, and there is little outgassing inthe vacuum chamber. Touching the pen to the region of interest wouldalter the surface, so the ink is applied near the region of interest,and the ink then wicks onto the region of interest. Compounds in the inkprotect some surfaces. The area affected by the felt tip is very largecompared to the sub-micron features of modem integrated circuits, andthe positioning accuracy of the ink is insufficient.

The industry needs a method of rapidly and accurately applying alocalized protective layer without damaging a work piece surface.

SUMMARY OF THE INVENTION

An object of the invention is to provide a local protective layer forcharged particle beam processing.

In accordance with the method, liquid is directed to a region ofinterest on the work piece surface. The liquid dries on the surface toprovide a protective layer. In a preferred embodiment, the surface isnot touched by the applicator. For example, a drop of liquid can beejected from a source and directed towards the work piece.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more through understanding of the present invention, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram showing a preferred embodiment of the presentinvention.

FIG. 2 is a flow chart showing preferred steps of using the system ofFIG. 1.

FIG. 3 shows an embodiment of the invention in which an ink jet systemis positioned outside the vacuum chamber of a charged particle beamsystem.

FIG. 4 is an enlarged view of the system of FIG. 3.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the invention uses an ink jet-type dispenserto locally apply a drop of fluid to a work piece. The fluid dries toform a protective layer. For example, the fluid can be a hydrocarbonbased ink or it could be a colloidal solution containing, for example,silver.

Embodiments provide for the high throughput application of a localizedprotective layer. Embodiments also provide damage-free deposition of aprotective layer. The invention provides flexibility in choice ofprotective layer - the layer material can be changed by changing thefluid source. A system in accordance with the invention allows fordeposition of the protective layer outside of the vacuum chamber, sothat the protective layer can be applied to one work piece while asecond work piece is being processed by the charged particle beamsystem.

In some embodiments, an ink jet head similar or identical to those usedin ink jet printers can apply the fluid. The type of fluid being appliedcan be readily changed by changing ink cartridge. The fluid source couldbe an ink cartridge, and can be integrated with existing front endsystem of the charged particle beam as shown, for example, in FIG. 3.

The fluid typically comprises a liquid carrier having a solute orsuspension contained therein. After the fluid is applied, the dropdries, for example, by evaporation of the liquid carrier or by achemical alteration, such as hydration. A preferred protective layer issufficiently conductive to dissipate any electrical charge produce bythe impact of the charged particle beam onto the work piece. A preferredprotective layer is “vacuum friendly,” that is, it does not “outgas” orcontinue to evaporate in the charged-particle beam vacuum chamber tointerfere with the charged particle beam or contaminate the work piece.A preferred protective layer stabilizes the structures on the workpiece. The preferred protective layer does not interact with or alterthe structures on the work piece and provides mechanical strength sothat the dimensions of structures changes little or not at all under theimpact of the charged particle beam.

A preferred embodiment deposits a small drop of a fluid onto the workpiece, and the drop that dries to form a protective layer. The drop canbe applied outside of the vacuum chamber, and the fluid can dry beforebeing place in the vacuum chamber. In some embodiments, a low volatilityfluid that dries by chemical change, rather than by evaporation, can beapplied within a vacuum chamber.

Technologies to provide an accurately placed, small drop are known, forexample, from ink jet printer technology. There are several welldeveloped ink jet technologies. Thermal ink jet printers useelectrically heated ink chambers to rapidly form a bubble that propelsink from a chamber. The fluids used in a thermal printer must haveproperties suitable for rapidly forming a bubble, without leaving behindcontamination. A continuous ink jet uses a high pressure pump to expelthe ink, which is typically broken into drops by a piezoelectriccrystal. An electrode applies a controlled electric charge to the drops,which are deflected by a second electrode. Lastly, in piezoelectricprinters, an electrical current causes a piezoelectric crystal torapidly deform to propel a drop of ink from a chamber. Piezoelectricprinter can accommodate a wider variety of fluids than can a thermal inkjet printer.

An ink jet printer is capable of creating drops having volumes on theorder of picoliters. The spot size for the local protective layerapplied by the drop preferably has an area of less than about 100 μm²,more preferably less than about 50 μm², and most preferably less thanabout 10 μm². Modern ink jet printers are capable of producing a spotsize having a diameter of less than 4 μm.

The smaller the spot size, the greater the placement accuracy isrequired to ensure that the region of interest is covered by theprotective layer. Ideally, a system uses the smallest drop size thatreasonably ensures coverage of the region of interest at the availableplacement accuracy. Modem ink jet printers are also accurate to with 3μm in drop placement. To place the drop, an optical microscope image canbe used, together with image recognition software, to align the ink jetcoordinate system with the work piece coordinate system. Imagerecognition systems are available, for example, from Cognex Corporation.

Inexpensive inkjet technologies provide individual spot placement thatcan be controlled tightly to within 20 μm with a drop volume of 4 pL (4μm³). Thermal dye deposition yields similar placement with slightlylarger drop volumes, while offering significantly greater flexibility indeposition material composition. Thermal dye deposition, also calleddye-sublimation, is a process of thermally evaporating a small amount ofthe dye/protective layer, and then the evaporated material deposits ontothe target, not going through a liquid phase. Current practices requireFIB related damage (deposition and Ga+ contamination) levels to belimited to within a 150 μm radius of the process site. The positioningaccuracy and droplet volumes mentioned above should satisfy thisrequirement.

If a colloidal suspension is used to deposit the protect layer, theparticles sizes, for example, of silver, are preferably less than about100 nm, more preferably less than about 50 nm, and most preferablybetween about 1 nm and about 10 nm. Silver is a preferred materialbecause it does not react with a semiconductor work piece surface and isconductive.

Another useful fluid for applying a protective layer compriseshydrocarbon-based inks.

Preferred protective layer application solutions not only provide aprotective layer having the desirable properties described above, apreferred solution should be capable of being used in the applicationsystem described above. As described above, different ink ejectionsystems may be require certain properties in the fluids used.

FIG. 1 is a block diagram showing a preferred protective layerapplication system 100 for use with a charged particle beam system 101.FIG. 2 is a flow chart showing the steps of a preferred method of thepresent invention. In step 202, a precision stage 102 capable ofprecisely positioning a work piece, such as a semiconductor wafer 104,moves the wafer 104 under a camera 106 that is part of an opticalrecognition system including a computer 110. In step 204, the opticalrecognition system 106 forms one or more images of the work piecesurface and in step 206, the image is analyzed by image recognitionsoftware 107 to recognize reference points on the wafer. In step 208,design information from the wafer is used to correlate the coordinatesystem of the wafer as determined from the image with the coordinatesystem of the stage 102. In step 210, the stage is moved so that thearea of interest of the wafer 104 is under an ink jet type print head108. In step 212, a drop of fluid 112 is propelled from the ink jet typehead to the wafer. The fluid is supplied by a reservoir 114 or by acarrier, such as the carrier film used with dye sublimation system (notshown). If necessary, the fluid is allowed to dry in step 214. Skilledpersons will recognize that in a dye sublimation system, the fluid thatis the source of the protective layer is a gas that condenses as a solidonto the work piece, and thus does not need to dry. The wafer isinserted into a charged-particle beam vacuum chamber in step 216. Instep 218, a portion of the region of interest is processed by chargedparticle beam processing. In some embodiments, rather than moving thewafer on a stage to position the region of interest under the ink jet,the ink jet is moved to a position above the region of interest. Inother embodiments, the ink jet is aligned by a combination of moving theink jet and moving the wafer.

FIG. 3 shows a work station 300 at which a simulation of a robot waferhandler 302 removes a wafer 306 from a cassette 308 and places the waferunder an ink jet head assembly 310, which includes a support 312 uponwhich an ink jet head 314 can move or is fixed. FIG. 4 shows an enlargedpicture of the work station of FIG. 3.

Although the present invention and its advantages have been described indetail, it should be understood that various changes, substitutions andalterations can be made herein without departing from the spirit andscope of the invention as defined by the appended claims. For example,although liquids and gases are described above, a fine powder may beapplied as the protective layer. A fluid as herein means anything thatflows, including a liquid, gas, or a fine powder. A fluid does notinclude a charged particle beam, such as a beam from an ion or a clustersource. Moreover, the scope of the present application is not intendedto be limited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

1. An apparatus for providing a protective layer on a work piece,comprising: a support for positioning a work piece; an applicator forlocally applying a fluid onto the work piece to form a local protectivelayer to protect an area on the work piece during charged particle beamprocessing, the applicator applying the fluid without touching the workpiece surface to which the fluid is applied; and a charged particle beamsystem for operating on the work piece, the charged particle beam systemprogrammed to process a localized area of the work piece onto which theprotective layer was formed.
 2. The apparatus of claim 1 which theapplicator comprises an ink jet head.
 3. The apparatus of claim 2 inwhich the ink jet head comprises a thermal ink jet head, a piezoelectricinkjet head, a continuous in jet head, or a thermal sublimation-type inkjet head.
 4. The apparatus of claim 1 in which the fluid comprises aliquid that dries on the work piece to deposit a protective layer. 5.The apparatus of claim 1 in which the fluid comprises a colloidalsuspension and in which the applicator includes a reservoir containingthe colloidal suspensor.
 6. The apparatus of claim 1 in which the fluidcomprises a hydrocarbon based ink and in which the applicator includes areservoir containing the hydrocarbon based ink.
 7. The apparatus ofclaim 1 in which the fluid comprises a gas that forms a solid on thework piece.
 8. The apparatus of claim 1 in which the applicator iscapable of applying a drop of liquid having a volume of less than 20 pLwith a positional accuracy of 100 μm.
 9. The apparatus of claim 1further comprising: a camera for forming an image of the work piecesurface; and a computer executing a program for recognizing an image onthe work piece and for aligning the ink jet head with a feature on thework piece.
 10. An apparatus for providing a protective layer on a workpiece, comprising: a support for positioning a work piece; an applicatorfor locally applying a fluid Onto the work piece to form a localprotective layer to protect an area on the work piece during chargedparticle beam processing, the applicator applying the fluid as a gasusing a sublimation process; and a charged particle beam system foroperating on the work piece, the charged particle beam system programmedto process a localized area of the work piece onto which the protectivelayer was formed.
 11. A method of providing a protective layer onto awork piece for charged particle beam process, comprising: directing afluid toward the an local portion of a work piece surface, theapplicator of the fluid not touching the area of interest and the fluidsolidifying to form a protective layer; and directing a charged particlebeam toward a portion of the work piece surface covered by theprotective layer.
 12. The method of claim 11 in which directing a fluidtoward a local portion of a work piece surface includes directing acolloidal suspension toward the work piece surface.
 13. The method ofclaim 12 in which directing a fluid toward a local portion of a workpiece includes directing a fluid including a colloid of silver onto thework piece.
 14. The method of claim 12 in which the average particlesize in the colloid is less than 100 nm.
 15. The method of claim 12 inwhich the average particle size in the colloid is less than 50 μm. 16.The method of claim 11 in which directing a fluid toward a local portionof a work piece surface includes directing a hydrocarbon based ink onthe work piece.
 17. The method of claim 11 in which directing a fluidtoward a local portion of a work piece surface includes directing a drophaving a volume of less than 20 pL.
 18. The method of claim 11 in whichdirecting a fluid toward a local portion of a work piece surfaceincludes depositing a protective layer onto an area having a dimensionof less than 10 μm.
 19. The method of claim 11 in which directing afluid toward a local portion of a work piece surface includes depositinga protective layer onto an area having a dimension of less than 10 μm.20. The method of claim 11 in which directing a fluid toward a localportion of a work piece surface includes applying a voltage to apiezoelectric crystal to propel a drop of fluid toward a local portionof a work piece surface.
 21. The method of claim 11 in which directing afluid toward a local portion of a work piece surface includes applying avoltage to heat the fluid to propel a drop of fluid toward a localportion of a work piece surface.
 22. The method of claim 11 in whichdirecting a fluid toward a local portion of a work piece surfaceincludes thermally evaporating a small amount of material, the materialthen depositing onto the work piece to form a protective layer.